Organic electroluminescence (organic EL) is a light-emitting diode (LED) in which the emissive layer is a film made by organic compounds which emits light in response to an electric current. The emissive layer of organic compound is sandwiched between two electrodes. Organic EL is applied in flat panel displays due to their high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first observation of electroluminescence in organic materials were in the early 1950s by Andre Bernanose and co-workers at the Nancy-University in France. Martin Pope and his co-workers at New York University first observed direct current (DC) electroluminescence on a single pure crystal of anthracene and on anthracene crystals doped with tetracene under vacuum in 1963.
The first diode device was reported by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The device used a two-layer structure with separate hole transporting and electron transporting layers resulted in reduction in operating voltage and improvement of the efficiency, that led to the current era of organic EL research and device production.
Typically organic EL device is composed of layers of organic materials situated between two electrodes, which include a hole transporting layer (HTL), an emitting layer (EML), an electron transporting layer (ETL). The basic mechanism of organic EL involves the injection of the carrier, transport, recombination of carriers and exciton formed to emit light. When an external voltage is applied to an organic EL device, electrons and holes are injected from a cathode and an anode, respectively, electrons will be injected from a cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from an anode into a HOMO (highest occupied molecular orbital). When the electrons recombine with holes in the emitting layer, excitons are formed and then emit light. When luminescent molecules absorb energy to achieve an excited state, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. 75% of the excitons form by recombination of electrons and holes to achieve a triplet excited state. Decay from triplet states is spin forbidden, Thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency. In contrast to fluorescence electroluminescent device, phosphorescent organic EL device make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescent devices from 25% to 100%. The spin-orbit interactions is finished by some heavy atom such as iridium, rhodium, platinum, palladium and the phosphorescent transition may be observed from an excited MLCT (metal to ligand charge transfer) state of organic metallic complexes.
The organic EL utilizes both triplet and singlet excitons. Cause of longer lifetime and the diffusion length of triplet excitons compared to those of singlet excitons, the phosphorescent organic EL generally need an additional hole blocking layer (HBL) between the emitting layer (EML) and the electron transporting layer (ETL) or electron blocking layer (EBL) between the emitting layer (EML) and the hole transporting layer (HTL). The purpose of the use of HBL or EBL is to confine the recombination of injected holes and electrons and the relaxation of created excitons within the EML, hence the device's efficiency can be improved. To meet such roles, the hole blocking materials or electron blocking materials must have HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels suitable to block hole or electron transport from the EML to the ETL or the HTL.
For full-colored flat panel displays in AMOLED or OLED lighting panel the material used for the phosphorescent dopant for emitting layer are still unsatisfactory in half-lifetime, efficiency and driving voltage. These organic metallic complexes still have disadvantages for industrial practice use. The phosphorescent dopant with preferential in-plane (horizontal) emitting dipoles are beneficial to optical out-coupling of OLED, since the ratio of vertical emitting dipoles contributing little to external emission is reduced and the radiation pattern of a horizontal emitting dipole is in general more suitable for optical out-coupling. Therefore, an emitter with proper substituents can be helpful to enhance the ratio of horizontal emitting dipole in the emission layer of OLED. Meanwhile, the proper substituents around an emitter can effectively block nearby electrons and holes, so that electrons and holes can easily recombine in the emitter and the efficiency of OLED can be improved.
In the present invention, for the purpose to prolong the half-life time and lower driving voltage for phosphorescent dopant in emitting layer for organic EL device, we employ an paracyclophane skeleton link to the ligand of the iridium complexes, then chelate with one or two bidentate ligand to finish the metallic complexes represented as general formula(1). The paracyclophane-based iridium complexes show good thermal stability and charge carrier mobility for organic EL device. Many prior-arts of iridium complexes such as U.S. Pat. No. 6,835,469B2, U.S. Pat. No. 6,916,554B2, U.S. Pat. No. 7,011,897B2, U.S. Pat. No. 7,429,426B2, U.S. Pat. No. 7,709,100B2, U.S. Pat. No. 7,851,072B2, U.S. Pat. No. 8,269,317B2, U.S. Pat. No. 8,492,006B2, U.S. Pat. No. 8,519,384B2, U.S. Pat. No. 8,557,400B2, U.S. Pat. No. 8,778,508B2 et al. But there are no prior-arts demonstrate an paracyclophane skeleton link to iridium complexes used as phosphorescent dopant of emitting layer for organic EL device.